Telephone system



June 11, 1935.

O-HOR/ZONTAL GROUP C PR/MARY INTERMEDIATE GROUP OF TEI VS w. w. CARPENTER 2,004,599

TELEPHONE SYSTEM Filed July I9, 1933 a Sheets-Sheet 1 F IRS T 1 GROUP OF TENS mm a INVENTOR W W CARPENTER ma E FIG. 1

AT TORNEY June 11, 1935. -w. w. CARPENTER 2,004,599

' TELEPHONE SYSTEM Filed July 19. 1935 S SheetS-Sheet 2 n E k) \l E E 6 I El m A T TORNEV FIG. 2

June 11, 1935;

w. w. CARPENTER TELERHQNE SYSTEM Filed July 19', 1935 a Sheets-Sheet I5 INVENT 'OR By M! w. CARPENTER m wt A T IORNEY I June 11, 1935. w w, CARPENTER 2,004,599 I TELEPHONE SYSTEM Filed July 19, 1933 8 Sheets-Sheet 4 mum I "cum-Mm w E i a 5 1* INVENTOR m m CARPENTER ATTORNEX J 1935. w w. w. CARPENTER 2,004,599

TELEPHONE SYSTEM Filed July 19, 1933 8 Sheets-Sheet 5 O'SENDER SELEmR SELECTOR WWW /N VE N TOR m w CARPENTER y hfim .4 T TORNEY June 11, 1935. 7 w. W.'CARP ENTER 2,004,599

TELEPHONE SYSTEM Filed July 19. 1933 8 Sheets-Sheet 6 o-snvasn SELECTOR 4- SENDER SELECTOR GROUP INVENTOR By M. M. CARPENTER,

A T TORNEY June 11, 1935; w. CARPENTER TELEPHONE SYSTEM Filed July 19, 1933 8 Sheets-Sheet 7 MzHH MQSQQQSQ INVENTO)? W W CARPENTER.

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A T TORNE Y Iune 11, 1935. w. w. CARPENTIER TELEPHONE SYSTEM Filed July 19, 1953 8 Shets-Sheet 8 4 (88 @8838 xkxikm 8 INVEN TOR W. W CARPENTER ATTORNEY its object an increase in the traffic carrying Patented June 11, 1935 TELEPHONE SYSTEM Warren W. Carpenter,

Garden City, N. Y., as

signor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 19, 1933, Serial No. 681,102

23 Claims.

This invention relates to telephone systems and more particularly to a link circuit for extending subscribers lines to switching trunks and to register senders. The invention has for eniciency of the switching and registering apparatus required for establishing the connections.

Owing to the relatively small capacity of the switches used in automatic telephonesystems, the subscribers lines and trunks are divided into small groups each having access to a particular group only of all the outgoing trunks of the corresponding switching stage. It is known that this grouping of lines increases the total number of trunks required to handle the trafic and, consequently, reduces the trafiic carrying efficiency of the trunks themselves because of the greater tralflc variations in small groups. It is also recognized that the larger the groups of trunks worked in common, the more eificient is the service and the fewer the total number of trunks required to carry the traffic. The same, of course, is true of register senders.

It has been proposed to improve the efiiciency of trunks and senders by interconnecting an additional switching stage between subscribers lines and outgoing trunks thereby giving subscribers lines access to a larger number of outgoing lines and an additional switching stage between trunks and senders without increasing the capacity of the connecting switches. The known double pre-selecting system is an example of this method of increasing the efficiency of the trunks without increasing the capacity of the switches and my invention is concerned with further increasing the eificiency of such systems and further extending the principle thereof to register senders.

In accordance with the invention, therefore, one specific embodiment of which is disclosed herein by way of illustration this is accom plished in the following improved manner:

The link circuit which is the subject of this in vention uses a switching medium of known construction of the coordinate type. It comprises a primary selector and a sender selector, the former being arranged to connect subscribers lines arranged in main groups and subdivided into horizontal groups of convenient size in the known coordinate manner, with outgoing trunks accessible through the primary selector, and the latter being'arranged to connect an available outgoing trunk with an idle sender which is accessible through the sender selector. Each link is accessible to a particular horizontal group of lines and is associated with other links inthe same horizontal group through a horizontal lock-out circuit which controls the 'extensionof calling lines from its ownshorizontal group, and.- also a common vertical'lock-out circuit for controlling the extension-of calling lines from different horizontal levels.

The primary selector comprises a connecting relay which has access to a horizontal group of lines and to a plurality of outgoing trunks. If, for example, a decimal arrangement is-assumed for convenience and then primaryselectors to each horizontal group of lines are provided and each prnnary selector, in turn, with accessibility to ten outgoing trunks, it is evident that each horizontal group of subscribers lines has an overall accessibility to one hundred outgoing trunks, the same trunks, of course, being multiplied to the primary selectors of other hori-. zontal groups of lines. By using the subscribers lines in larger main groups than is now possible with mechanicalline finder frames of knownconstruction, and outgoing trunks also in larger groups, greater efliciency in the use of the equipment can be realized and, therefore, less apparatus needed for carrying the same traffic load.- The sender selector is generally ofthe same construction as the primary selector and, in the present embodiment of the invention, comprises a group of connecting relayseach havingacces's to a plurality of groups of trunks and to a group of senders through each sender selector. The groups of trunks are multipled to as many sender selectors as are necessary by traffic conditions and'each sender selector'to which-saidtrunks are extendedv increases the number of senderswith which said trunks may establish connection by the number of trunks available through said selector. The senders may be extended to as many sender selectorgroups as maybe required for the most efiicient handling of the trafic.

One novel feature oi my invention is the correlation of each group of ten outgoing trunks with a particular group of sender selectors that is, the selection of an outgoing trunk in a group reached through a particular primary selector involves the simultaneous selection of an idle sender in the group of sender selectors correlated with the particular group of outgoing trunks containing the chosen trunk. If, in fact, a trunk is available while a sender selector is not available in the correlated sender selector group then, after a momentary interval, selections will be made in another available;

' the correlated groups of trunks and senders are heavily loaded up with trairic from all the main groups.

In general, my invention works in the following way: When a calling subscriber in any hori-' zontal group initiates a call, the horizontal and vertical group lock-out circuits operate to extend the calling line to one of the ten primary selectors through each of which a, group of outgoing trunks may be reached. Each primary selector is tested for its idlei or busy'condition and all the idle primary selectors cause the operation of corresponding test relays which, in turn,"close through the respective group control circuits of the outgoing trunk groups associated with each of the idle primary selectors. These control circuits are each serially extended through the test circuit of the correlated group of sender selectors and should no sender selector be'available in any or all of the correlated sender groups,- then the corresponding group control circuits remain open and the selection ofa-trunk and sender is restricted to a preferred trunk group Whose correlated sender selector-group does contain an idle sender selector and available senders thereover.

' Theselection of a preferred trunk group from all available trunk groups is made by simultane' ouslyandseparately making a trunk group test of all said available groups. than one 'idle trunk in' each group then the group selected is the one in the highest order of preference. trunks or all the trunks but one in each group are-busy, the entire group tests as if all the trunks within it were busy. Under such circumstances the failure to lock-in a trunk group during the trunk groups test causes the trunk group test of all said groups to be canceled and the'groups are then retested to select the one idle trunk in a preferred group which had previously' tested all busy but which, in fact, did contain an idle trunk.' In this way all groups are tested so as to spread the flow, of traffic as evenly aspossible through all groups.

The sender selector operations in any'or all groups are so much like what has already been described with reference to the primary selectors that it is hardly necessary to repeat it in this brief digest, it being readily understood Fig. 4 shows a skeletonized outgoing trunk selector of the panel type, and a diagrammatic presentationof a group of senders.

If there is more If, on the other hand, all the Fig. 5 shows the first group of sender selectors with associated trunk and sender groups.

Fig. 6 shows the last group of sender selectors with associated trunk and sender groups.

Fig. 7 shows the sender selector control circuit.

Fig. 8 shows a diagrammatic representation of the relationship'which exists between subscriber primary selector, sender selector and .sender groups.

Fig. 9 indicates how Figs. 1 to 8 should be arranged with respect to each other in order to completely disclose the invention.

Before proceeding to a detailed description of the particulars of the invention, it may be well to consider it somewhat in general terms from a study of Fig. 8.

- For purposes of illustration it may be assumed thatFig. 1 represents a main group of 1000 subscribers lines. ten groups of lines each and each group arranged for'connection to any one of ten primary selectors. Since there' are ten groups of lines, there will be ten horizontal groups of lines and selectors, namely the zero horizontal group up to and including the ninth horizontal group. It will be further assumed that there are 100outgoing trunks which may be used in common by said group of 1000 lines and also by other similar groups if the quantity of the trafiic warrantsit. This main group of 100 trunks is divided into ten separate subgroups of ten trunks each and each trunk within each subgroup is ar-:

the same corresponding selector in each group,

which is the way the trunks are shown for connection in Fig. 8.

But While the lines and primary selectors are arranged in ten horizontal groups, the sender selectors are arranged in five-horizontal groups composed of ten sender selectors to the group; To each of these horizontal groups of sender selectors are extended two trunk subgroups available to the 1000 subscribers and also to other pairs of trunk subgroups available to other groups of 1000 subscribers. zero horizontal group of sender selectors may be connected with trunk subgroup zero and trunk subgroup 9, the third horizontal group with trunk subgroups 2 and 1, while the fourth or last with trunk subgroups 4 and 5. This distribution is, of course, not intended to limit the invention to the arrangement as shown, but sim ply indicates that where the traflic density permits it, the number of trunk subgroups terminating on any'horizontalgroup of sender selectors may be increased ordiminished as desired.

It might be possible, for example, to have ten groups of sender selectors and terminate one trunk subgroup over from each group of lines on one sender selector group."

The main group of senders is available to all sender selectors, and is arranged for connection amongst them in the same way as the trunks were arranged amongst the primary selectors; that is, each sender is connectible to one sender These lines are subdivided into For For instance, the

selector of a horizontal group of selectors and to each corresponding selector in each of the other horizontal groups of sender selectors.

Obviously, the connection of a calling line in any horizontal group with an idle trunk and an idle sender must take place through the intermediate agency of a primary selector and a sender selector. Calling line A, for instance, may establish a connection with an idle sender only through the selection of an idle primary selector which has accessibility to a trunl; in a subgroup whose correlated group of sender selectors also contains an idle sender selector which has accessibility to an idle sender. If a sender selector and a sender is not available, then the calling line must select an idle trunl; from some other trunk subgroup accessible through the primary selectors but which, at the same time, can obtain an idle sender selector and an idle sender through its own correlated group of sender selectors and senders.

A detailed description of the system will now be given with reference to the remaining fig ures of the drawings.

When a subscriber A in any horizontal group, the Q-group, for instance, removes his receiver from the hook, a circuit is closed for relay are which extends from battery through the winding of relay lit, the inner contacts of relay I'll, subscribers loop, to ground through the outer contacts of relay ill. The operation of relay no causes a circuit to be closed extendingfrom ground on the left contacts of relay 33%, conductor 395, outer contacts of relay ill), back contact of relay I51], lower winding of relay E68, conductor I12, back contact of relay 381, contacts of relay 389, winding of relay 393 to battery. Relay EEG operates in this circuit and locks over another circuit extending from battery through its top winding and bottom front contact, conductor l'l, over a chain path including the bottom back contact of each relay corresponding to relay 569 in the successive horizontal groups including the bottom contact of relay 259 in the 9th or last horizontal group, conductor 2W, winding of relay contacts of relay 389, winding of relay 393, to battery. Relay 558 further closes a circuit from ground on its inner upper contact, conductor 25!, windin of relay 5MB to battery. Relay 355i operates and performs functions which are described hereinafter. Another circuit is also closed from ground on the top middle contact of relay lfill, to the windings in parallel of all the tens group relays l80-l5fi of which there are as many in each horizontal group as there are minor subdivisions of ten line groups in each of said horizontal groups. Relays mi Isl! operate and since the calling line A belongs to the first group of ten lines controlled through relay Hill, said relay closes a circuit whose path and function are also described hereinafter.

Relay 38'! operates in the locking circuit of relay E80, opens the operating path of relay I60 in each horizontal group-s0 that said relays are prevented from operating after one of their number has operated and locked in series with relay 387, closes an obvious circuit to the winding of relay 388 which operates to perform other functions that are noted hereinafter, and further closes a group offive separate circuits to the relays 32li324 extending from battery through the winding of relay 393, contacts of relay 389, inner front contact of relay 381, in

parallel through each of the windings of relays 320-324 to ground for each of said circuits on the top outer contacts of relays 3l5--3l9 respectively. Relays 326-4324 operate and remove the short circuit around resistances, such as 326 and 326 which are used for making primary selector and trunk tests as described hereinafter. Relay 393 is marginal and slowto-operate and does not operate in the above named circuits. It operates only if, due to some trouble condition, two horizontal group relays, say relaysllifl and 269, should remain operated at the same time after an interval long enough to have caused the opening of their operating circuits through the contacts of relay 3&7. Una der such circumstances, the current flowing through the two separate circuit branches and combined through the winding of relay 393 will be sufficient to energize said relay, whereupon a circuit is closed to an alarm circuit (not relay Hill of the El-group, or relay 269 of the,

9th group, would be operated by circuits similar to the circuit of relay Hill described above. Since, however, the locking circuit of each preceding relay is controlled through the normally closed contacts of each succeedingv relay, it follows that although the group lock-out relay in each group may operate in response'to the initiation of simultaneous calls in their respective horizontal groups, yet only the look-out relay which is electrically nearest to relay 381 can possibly lock; that is, the relay whose locking path through the winding of relay 381 is undisturbed by the open back' contacts in the chain of previous relays which may have been operated. The breaking of the locking circuit at any point will cause all horizontal group relays to release except the one which has been successfully locked. All the other horizontal group relays will then await their turn on the re-estab lishment of the operating circuit, which occurs when the calling line in the horizontal group which has been locked in has been connected to an outgoing trunk, as more completely, described hereinafter. 7

Assuming, however, that relay Hill of the first horizontal group has been locked in and that no line in any other group, or in the same group, is calling simultaneously with line A, then a circuit is closed to the winding of relay I56 which extends from battery through the winding of said relay, top outer contacts of relay Mil), conductor I'M, lower'outer contacts of relay 388 to ground. Relay I59 operates, opens the path from the contacts of line relay I'll! to the operating winding of relay M9,, and locks through its inner contacts, conductor l'l5,.to ground on the lower inner contacts of relay 388. Ncw,'at the time relayv fist! operated, relay wt was also operated as already described. Consequently, a circuit isthen closed extending from ground on conductor 39% contacts of relay no, No. B contact-set of relay let, conductor.

I16, upper winding of relay 230 to battery. Re-

lay 230' operates, locks infa circuit extending from battery through its bottom winding and bottom front contacts, over a chain path including the bottom back contacts of other relays 23l230, one of which is furnished for each vertical row of subscribers lines in each tens horizontal sub-group, winding of relay 250,

which is a common relay for ten successive ver-' tical rows of lines, back contacts of similar intermediate relays, back contact of the end relay 253", conductor 252, top contacts of relay 388, to ground. Over its top contacts, relay 230 closes an obvious circuit to the windings of all vertical selecting magnets, 220-420 in the first vertical row of the first tens subgroup; that is, the one which contains. the calling line. These magnets operate and position the wire bar of the switch to lock' the cross contacts at the appropriate time, as more completely described hereinafter.

It will be observed that. the ground for the locking circuit of each of the relays 230-239 and the winding of each of the relays 250-256, one of which is in the above mentioned locking circuit of a relay in the appropriate group, is taken over a series chain of back contactsof each of said relays 250250. In the present embodiment of the invention, the subscribers lines are divided into ten main horizontal groups. Each group is further subdivided into a plurality of subgroups of ten lines each, and provided with a group' relay, such as relay l for the first subgroup of ten lines in the 0-horizontal group, through each of whose ten sets of contacts a calling line served by a particular subgroup relay may be further extended into the control circuit. Consequently,if there are simultaneous calls in different subgroups of the same or different horizontal group, the operation of the lock-out relay in. the group 250-250 in series with the most preferred relay in a group of relays 230239 cuts off the operating circuit of all succeeding relays 25025!] and the locking circuit of all relays in the preceding groups 230-239, by virtue of the fact that the operating ground of each preceding relay is cut oil at the back contacts of each succeeding relay. By means of. this arrangement, only one relay in the group 280-289 can operate as described hereinafter, and all horizontal groups containing calling lines must wait until the calling line in the most preferred horizontal group has been connected to an outgoing trunk before the lines in the less preferred groups can be extended for further operation. In this manner, successive horizontal subgroups are locked out to await their turn in accordance with the preference arrangement.

In the meanwhile, the operation of relay 250 closes a circuit for relay 280 extending from ground on the top contacts of relay 388, conductor 252, back contacts of relay 250*, back contaet series path of intermediate relays in the group of relays 250250 front contacts of re lay 250, winding of relay 2010 to battery. Relay 28D operates and closes a path to relays I90, 200, etc. of each horizontal group. Since a line in the il-horizontal group is calling while all other groups are locked out, a circuit is completed for relay [90, extending from battery through the winding of relay i90, conductor I'll, No. 0 contact set of relay 280, conductor 25l, top inner contacts of relay [60 to ground, and in parallel therewith over conductor 25!, winding of relay 3!!) to battery. Relay I operates,

and partially closes through each of ten paths to the ten separate horizontal magnets of the first subgroup of lines in the 0-horizontal group which contains the calling line. The circuit of the appropriate horizontal magnet is not completed, however, until a primary selector, an idle outgoing trunk accessible through said selector and an idle sender connectible to said outgoing trunk have all been selected, as more completely described hereinafter.

It will be recalled that relay 3l0 operated at the time relay I60 operated. Relay 3|0 partially closes circuits through the windings of the horizontal magnets of a group of ten primary selectors connecting with the whole group of outgoing trunks all of which are accessible to the lines in the il-horizontal group through said ten primary selectors, as more completely described hereinafter,

Now, each primary selector provides accessibility to a subgroup of ten outgoing trunks and the test for an available trunk, therefore, is divided into two separate but simultaneous tests; first, the test for an available primary selector and secondly, a test for an idle trunk accessible through the available primary selector. Each busy primary selector is arranged. to have its horizontal magnet operated, and since the winding of this magnet is connected directly to the sleeve conductor of the associated selector, the ground on this conductor also furnishes the test of the availability of the selector. Consequently-when relay 3I0 operates, each of the sleeve conductors of each of the primary selectors in the groupof the calling line is extended through the contact sets of relay 3l0 to a winding of a test relay. These relays are 330 to 339, inclusive. Idle primary selectors have no ground connected to the sleeve conductor and hence, upon the operation of relay 310, a circuit is closed for each idle primary selector from battery through the winding of the horizontal magnet of each idle primary selector to the winding of the test relay. A typical circuit is that for the O-primary selector which extends from battery through the winding of the selector horizontal-magnet I I0, conductor I18, No. 0 contact set of relay 310, winding of relay 330, to ground. Relay 330 operates but the quantity of current flowing through the circuit is not suiiicient to permit the magnet to operate. Each test relay in the group 330-330 corresponding to an idle primary selector thus operates in the same manner.

Now the outer contacts on each of what may be designated the first pair of test relays, namely, relays 330 and 339, are connected to the windings of a relay 315, while those of relays 335 and 336, the last pair, are connected to the windings of relay 3I9, the contacts of inter-' mediate pairs of test relays (not shown) being connected, respectively, to the windings of the intermediate relays 316, 3!! and 3I8 (not shown) in the group of five relays 3l53|9. Consequently, those test relays which have 0perated as a result'of the idle condition of the primary selector to which they were connected, further close circuit paths from ground through their bottom outer contacts, top winding of relay 3l5, for example, if it be assumed that the 6 or 9th primary selector or both are idle and that, in consequence, relay 330 or 339 or both have been operated, conductor 399, contacts of any normal horizontal magnet of the group of senderselectors .0 to 9, in the U-horizontal group of sender selectors, as, for instance, contacts of magnet 5H3, conductor 501, back contacts of relay 189 which relay is normal it any sender available through the 9th sender selector of the fl-horizontal group is idle, as more completely described hereinafter, conductor 582, back contacts of relay 380, to battery. Relay 3l5 operates in this circuit and looks over its lower winding and lower inner front contacts, over the series chain of back contacts of other relays identical with 315, such as 3E5 associated with other main district groups which have accessibility to the same subgroup of senders as more completely described hereinafter, winding of relay 38%, to battery. Relay 38B operates in this circuit and, by opening conductor 5B2, opens the operating circuit of every like relay in the chain sis-else. The operation of relay 3E5 further .opens the circuit of relay 32% but. this relay, being slow to release, does not release immediately. In the meanwhile, and during the interval in which relay 32s remains in its operated position, relay 315 causes a circuit to be closed from ground, through resistance 326 and inner contacts of relay 335i, if this relay has operated in response to the idle condition of the il-primary selector, top inner contacts of relay 3H5, winding of relay 310, top middle back contact of relay 325, in parallel through the upper winding of each of the relays 3M! to 349, conductor 39'! of relay 349 and a similar conductor on each of the other relays in said group of relays 3dfi3 i9, contacts of cam 58, to battery in each or" the outgoing trunk selectors correlated to the ten relays oi the group of relays 3:10-349. If each selector is idle, then each of their respective sequence switches are in position 3 and all relays 345 to 349, inclusive, operate. On the other hand, if any selector is busy, its associated sequence switch will have been advanced beyond position 3 in the well known manner and a circuit will not be closed through the winding of its corresponding relayin the group of relays 348-3 19. Assuming, however, for purposes of illustration, that the district selector shown in Fi 4 and associated with relay 3% is idle and in position 3, while at the same time other trunks in the same group are also idle, then the circuit through relay 3% and others in the group corresponding to said idle trunks is closed and if two or more trunks are idle then relay 310, which is marginal, operates but the quantity of current flowing through the separate test relays 3fl.8349 is not sufficient to cause said relays to operate. Relay 310, on operating, closes a circuit extending from ground on the right back contacts of relay 386, contacts of relay. 3Y8, lower winding of relay 360, to battery causing relay 363 to operate and lock in a circuit extending from battery through its top winding and top inner front contacts, winding of relay 886,. contacts of relay 39! to ground on the middle contacts of relay 35?. The operation of relay 38$, opens the operating circuit of relay 3G6 and closes a circuit from ground on its front contacts, bottom front contacts of relay 350 to the circuit path of relay 34! as already described. Relay 346 now operates and locks in a circuit over its bottom winding and bottom contacts, bottom contacts of relay 3P5, winding of relay 325, back contacts of relay 39! to ground on the contacts of relay 38-1. Relay 325 operates, opens the operating circuit of all relays in groups 3M3-349 and 350-359, and

closes a circuit for relay 30!] extending; from ground on the lower front contacts of relay 325,- winding of relay 390 to battery, causing said relay to operate. Another circuit is also closed extending from ground on the lower in-' ner contacts of relay 325, top outer contacts of the operated relay 340, No. 0, contacts of relay 3%, conductor 299, to the windingsin parallel or all vertical magnets of the first vertical group of outgoing trunks, that is, vertical magnets 2413', Mt, etc. These magnets operate and a, circuit is then closed extending from ground on the contacts of relay 3%, winding of relay,392, contacts of the last vertical magnet 240, conductor 2MB, top outer contacts of relay. 3%, conductor 3%, No. ilcontact set of relay IN, to the windingcf horizontal magnet Hill. This magnet now operates and since vertical'magnets 226-420 are already operated, the line contacts of the calling line'A areextended to the d-primary selector. Whenthe contacts. are closed, the ground which operated magnet we is further extended through its own contacts to the B-primary selector sleeve conductor H9, winding of G-primary selector horizontal magnet Hil to battery. Magnet H0 operates to close through the contacts of the ll-primary selector to the il-district selector shown in Fig. 4 thereby extending the calling line to said trunk. Relay 392 operates in the circuit of magnet m0 and causes the operation of relay 394 which, .in turn, operates relay 389. Relay 389 then opens the original starting circuit through relay 381, causing said relay to release, releasing thereby relay 388 and also unlocking relay E68. Relay 860 'releases, causes the release of relays E80, 180, etc. and relays E90 and i551. The release of-relay ltd opens the operating circuit of line horizontal magnet IEO but this magnet does not release since, in the meanwhile, and as described hereinaftenthe district selector to which the calling line has been extended has advanced toa position where ground is connected from the selector to conductor H9 thereby holding both the line and primary selector horizontal magnets [00 and H0 respectively, in an operated position. With the release of relay 388, relay 25E! releases, thereby releasing relays 23!], 280 and the line vertical magnets i2i322ll. The release of relay 289 releases relay 3! 6 thereby opening thecircuit of all selector vertical magnets and further releasing the relays in the group of relays 339 339 which-were operated in response to idle selectors. The line group and line st'art circuits are now all normal awaiting the initiation of another call in any of the horizontal groups.

It will be observed that in tracing the origination of a call from the line-station in-any horizontalgroup to the connecting primary selector and thence to the trunk selector, it has been tacitly assumed that 1) theii-primaryselector was available; (2) that two or more trunks connectible through said idle primary selector were idle, and (3) that the sender group correlated with said trunk group contained an idle sender.

The first assumption was indicated by the op eration of relay 330 in series withthe horizon the back contacts of relay 189 which is normal if two or more senders accessible through the sender selector correlated tothe normal horizontal magnet are idle asmore completely described hereinafter.

Obviously, there may be. a variation in any one or all of these factors. Theremay be, for instance, no available primary selector in the horizontal group to which the calling line belongs, -although the separate trunks connectible therethrough may all be idle. Also, the primary selectors may all be idle and yet all the trunks connectible therethrough may all bebusy; then,

again, While an idle primary selector and an idle connectible trunk the'reov'er may both be simultaneously available, yet the sender selector group having'access to the trunk group containing the idle trunk may have no available sender selector or no idle sender. In the first instance supposed the call may.v be either dismissed with an all busy signal transmitted to the calling subscriber, or said subscriber may be made to wait for a short interval until a primary selector is available; Each of the two last conditions mentioned, however, produces its own reaction by virtue of which the calling line is transferred to another primary selector with available idle trunks in trunk groups associated with sender selector groups containing free sender selectors and free senders thereover. Each of these separate conditions will now be considered in detail.

Primary selectors all busy When relay 3H] operates, one side of the horizontal magnet of each primary selector in the horizontal group of selectors available to the calling line-is connected to a test relaypmagnet MB of the D-primary selector being connected to the winding of relay 330 and magnet N9 of the 9th primary being connected to the winding of relay 339. If all -of the primary selectors are busy, then ground is connected to the Winding of each of their respective magnets, thereby short-circuiting the winding of each of the test relays 33B to'339, inclusive, none ofv which, in this case, will operate. Under such circumstances there is no primary selector outlet in the horizontal group to which the calling line is assigned and the subscriber willbesupplied with a suitable tone signal (not shown) indicating the absence of an outlet for the extensionof the line. The subscriber, upon hearing the tone, will restore the receiver and then try again, after a' brief interval;

More than one primary selector available er A initiates a call, primary selectors l to 8, inclusive, are busy and thatthe first and last, that is and 9th primary selectors are idle. Then, when the primary selector test isbeing made, primary selectors i to 8, inclusive, will have ground connected to their respective sleeve conductors while selectors ll and 9 will not.

Under such circumstances relays 33! to 338 inclusive, (only relays 335 and 335 being shown) of the'group 330 to 339 willnotoperate. Relays 330 and 339, however, will operate. The circuit of,relay* 330 has already been traced while=the-oircuit of relay -339 extends from battery through thewinding of horizontal magnet N9 of the Qthprimary selector, conductor I19, No. 9-contact set-of relay 3H1, winding of relay 339, to ground. Relays 33lland 339, in operating, close a previously described circuit for relay 3l5 provided that the group of sender selectors correlated to bothprimary selectors contains an available sender selector and idle senders connectible therethrough. If, however, both a sender selectorand a sender accessible thereover are not available simultaneously, the circuit of relay 3I5 is not completed in which case, since it has been assumed that the 0 and 9th primary selectors are the onlyones idle, all further operations aresuspended and the subscriber is supplied with an all busy tone in any suitable manner. i

In this connection it must be recalled, of course, that the'prefer'red embodiment of the invention shows a'pair of primary selectors correlated to one group of sender selectors, which, in turn is further correlated to other pairs of primary selectors assigned to other line groups. It is, however, just as possible to have a greater number of groups of sender selectors and to associate the trunk group 'or groups of but one primary selector with each group of sender selectors. Since there is one group of marking relays in the primary selector control circuit for each group of sender selectors-such as, for instance, relays 3I53l5 for the El-sender selector group and relays 3I9-.3l9 for the 4th sender selector group, with intermediate relay groups like 3|6-3i 6 etc. (not shown) for intermediate sender selector groups, then, an increase in the number. of sender selector groups to equal the number of primary selectors in each horizontal line group would necessarily call for additional marking relays for the added groups. Undersuch circumstances, each of two primary selector test relays, as for instance, relays 330 and 339, insteadof closing parallel circuits to the same sender group marking relay 3| 5, would each close a circuit-to a separate relay each of which would mark the sender selector group to which the primary selector in the line group tested by the relay is correlated. If these additional sender groups were supplied, then, in the illustration assumed, relays 33D and 339 would close circuits to. separate relays like relay 315 and if the correlated sender selector groups contained no available sender selectors or idle senders connectible therethrough, neither of these marking relayswould operate, in which event the calling subscriber would again receive busy tone as described.

It will be further observed that the marking circuit of each sender selector group in the primary selector control circuit comprises a group of relays 3l5-3l5 for each of said sender selector groups; The number of relays which comprise each marking circuit depends on the number :of primary selector frames using the same sender selector groups; there being one relay for each frame. Each of said relays is, in turn, controlled by the test relays of the primary selector frame which said relay marks. When therefore, a calling line in any of these other frames seeks access to an outgoing trunk in a group of trunks accessible over a particular idle primary selector, say the zero primary selector, the'test relay of said selector, corresponding to relay 330, will partially close the circuit of a relay, such as H in. the sender selector marking circuit of the sender selector group which has access to the trunk group containing the idle trunk. If the sender selector group contains an idle sender selector and an idle sender accessible thereover, relay 355 will operate and thereafter control further marking tests on its correlated frame as the already described operations of relay 3E5 control the marking operations on the frame under consideration.

Relay 335, in operating, closes a circuit for relay 31K! in parallel with the separate relays of the trunk group marked by the relay group 340 to 349, as already described, while relay ass operates and closes a circuit extending from ground, resistance 325, inner contacts of relay 339, top middle contacts of relay M5, winding of relay 379, top inner contacts of relay 325, windings in parallel of each of the relays in the group 35D359 which appertain to a trunk group of ten trunks accessible through the 3th primary selector, to battery on a cam corresponding to cam 55% of the trunk shown in Fig. 4, in each of the trunks in said group that are idle. Assuming that there are two or more idle trunks, then relay 319 operates and closes a circuit for relay 359 from ground on the right back contacts of relay 386, contacts of relay 3W, bottom winding of relay 368 to battery.

It will be observed at this point that the idle condition of the t-primary selector and the presence thereon of more than one idle trunk accessible therethrough resulted in the operation of relay 316 while the idle condition of the 9th primary selector and the presence thereon of more than one idle trunk accessible therethrough resulted in the operation of relay 319. These relays cause the operation of relay 3% and relay 369, respectively. But, inasmuch asthe locking circuit of each succeeding relay in the group of relays 38ll369 is controlled through the back contacts of a relay immediately preceding, only one of the relays so operated can remain locked. And the one which will remain locked is the one whose locking circuit is undisturbed by the operation of the preceding relays in the chain. In the case'here being illustrated, the locking circuit of relay 359 is opened at the back contacts of relay 380, while that of the latter is continuous through the winding of relay 38% as already described. Consequently, when relay 38% operates and opens the operating circuits of both relays 360 and relay 369, said relay 389 releases while relay 369 remains locked. With the operation of relay 38B, circuits are closed to operate the particulalr relays in the group 3:33 'l whose circuits are closed through the respective idle trunks to which they appertain.

Thus, if two or more primary selectors are both available and each of them contains idle trunks, that selector is chosen for the connection whose trunk group test relay, such as relay 366 for El-prirnary selector or relay 369 for the 9th primary selector, is nearest in electrical preference to relay 386. In the case under illustration, since it has been assumed that both the @-primary selector and the 9th primary selector were both idle as shown by the operation of their respective test relays 33B and 33$, and since both have two or more idle trunks available, as shown by the operation of relays 3 5i] and 379, then the ii-primary selector and an idle trunk accessible therethrough are chosen for connection to the calling line as already described. The number of relays in the group 340 to 3 39 whichwill operate depends, of course, on the" number of idle trunks, But the one which will remain operated and locked, thereby markingthat trunk for service, will be the one nearest in the preference in respect to relay 325, since, when the relays operate, a locking circuit for one of them will be closed extending from ground on the middle contacts of relay 33?, contacts of relayfiill, winding of relay 325,

bottom outer contacts of relay 325, seriesback contact chain in the group of relays Md to 34s, front contacts of relay 3 56, if it is assumed that the trunk it marks is idle, lower winding of relay 3% to battery. Relay 325 operates, opens the operating ch cuit of all trunk test relays in the group 3% to 3% which will now release except relay Std of the first trunk in the group which is locked in series with relay 25. Relay Silt operates as already described and a circuit is now closed for the primary vertical magnets as before. When the primary selector vertical magnet 246 operates, a circuit is closed extending from ground on the back contacts of relay E93, winding of relay E92, contacts of magnet 2M, conductor 2E8, top outer contacts of relay 36D, conductor 398, No. 9 contact set of relay i913, winding of horizontal magnet me to battery. Horizontal magnet it operates and closes through the contact wires of the calling line on the 9 horizontal path which extends to the 0 primary selector. The

operation of the horizontal magnet we further 7 extends its operating ground through its contacts, conductor H9, winding of the it primary selector horizontal magnet HE! to battery. Magnet ilil operates and extends the calling line to the 0 outgoing trunk selector, after which the line and start circuit release as before.

Should there be a plurality of primary selectors available, then the one which will be chosen is that which is nearest in electrical preferenceas determined by the series lock-out circuit of the primary test relays 3% to 369 as already described. However, as respects the available trunks accessible through each of the free primary selectors, the selection of a trunk is not necessarily made from that primary selector which is of a preferred choice should there be two or more idle primary selectors available for the connection. The choice of the primary selector, as between two or more avail--v able ones, depends under such circumstances, on which of them has more than one idle trunk available and of those which do have more than one idle trunk then the primary selector chosen is the one which is the earliest electrical preference. A primary selector which has but one idle trunk is chosen only when all other primary trunks also have but one idle trunk each and the primary selector so chosen is, under such circumstances, then again determined by the order of electrical preference. The object in this reserve feature is to spread the flow of traffic to the less loaded primary selectors in periods of heavy loads. Since the entire group of 100 trunks is available to a main group of calling lines subdivided into several horizontal" groups, each of which mayhave different tramc densities, it is obvious that some trunk groups would obtain an unequal portion of the traific while other trunk groups would be idle or less heavily loaded. Hence by arranging the order of trafiic distribution in a manner such that I relay 386.

ceeding call can be transferred to a primary selector which has more than one of its accessible trunks available, traiiic is distributed evenly throughout the entire trunk group. This feature works as follows: r

Let it be assumed for the purpose of illustration that primary selectors to 8 are again busy, that primary selector 9 has but one trunk available, letus say trunk thatprimary selector 9 has more than one trunk available and that further, the sender group correlated to said two primary selectors has available sender selectors and senders. Then, when subscriber A initiates a call, primary selector test relays 339 and 339 both operate, each in the manner previously described; and relay 3|5 operates in consequence of the availability of a sender selector and sender, and thereafter locks. Since the operation of relays 330 and 339 signifies that both primary selector 0 and primary selector 9 are available for extending the connection, two separate trunk group test circuits are closed, namely, that controlled by relay 360 and the individual test relays 390 to 349 testing the in dividual trunks available through. primary selector 9, and that controlled. by relay 369 and the individual test relays 359 to 3559' testing the individual trunks available through primary selector 9. Now, it has been assumed that primary selector 9 has but one trunk idle, that is trunk 0, marked by relay 349; hence a circuit is established extending from ground, resistance 32%, upper contacts of relay 330, top inner contact set of relay 315, winding of relay 319, top outer contacts of relay 325, top winding of relay 3 39 of the D-trunk, conductor 391, battery on cam 459 of the idle D trunk. Relay 310, however, is marginal in that, when connected in series with but one relay of the associated group of individual test relays 340-349 and resistance 326, the quantity of current flowingthrough circuit is not sufficient to operate it. Neither,- of course, does relay 349 operate.

But while the trunk group test circuit of the trunks available through primary selector 0 is in progress and results in the circuit operations above described, the group test circuit of the trunks available through primary selector .9 con currently operates, for a circuit path was simultaneously completed extending from ground, resistance 325, inner contacts 01": relay 339, bottom inner contacts of relay 315, winding of relay 319, top inner contacts of relay 325, through the separate windings, in parallel, of all relays 359 to 359 of the separate idle trunks in the group accessible through primary selector 9, to battery on cam 459 in each of the separate trunks. According to the assumption, this group contains more than one idle trunk and hence two or more parallel circuits are closed through the separate trunk marking relays in the group of relays 359-339 and the combined current, all flowing through the winding of relay 319 is sufiicient to cause said relay to operate. Relay 319 operates relay'369, which, in turn, locks in a previously described circuit which causes the operationof The operation of relay 386 then causes the closure of previously described circuits for operating the preferred relay in the group of relays 350-359 corresponding to an idle trunk and further causes the operation of relay 325-as previously described. Relay 369 also closes the circuit of horizontal magnet I199 in the manner described for horizontal magnet 10o therebylocking the calling line to primary selector 9 through which the line is further extended to the idle trunk appertaining to the relay in the group 350 to 359 which was locked in series with relay 325. In this manner, the selection of the one available trunk in a preferred primary selector is avoided if there are twoor more idle trunks accessible through any other available primary selector thereby distributing traffic evenly to all groups. v i

If it should happen that all trunk groups available through the separate idle primary selector each have but one trunk available, then the line is switched for connection to the idle primary selector which is first choice in the preference arrangement of primary selectors. This is accomplished as follows: Let it be assumed that allsender groups have available sender selectors and senders. Then, when all trunk groups accessible through each primary selector have but one trunk idle, and all corresponding primary selectors in the horizontal group containing the calling line are available, the primary selector test relays 339 to- 339 operate. Then the simultaneous group test conducted through relays 319 to 319 will cause a failure of each of these relays to operate as already described since each of these relays will not operate in series with a resistance such as'326 and one trunk marking relay. It will be observed, however, that at the time relay 381 operated, relays 329 to 324 inclusive, operated and their operation served toremove the short circuits which normally exist about each resistance such as resistance 325 controlled through aset of contacts on each of the primary selector test relays 339-339. It will also be remembered that the trunk group test is made with each of these short circuits removed. Now when all the relays 315 to 3l9, inclusive, operate in response to the availability of the sender selector groups which they mark, the circuits of relays 329 to 324 are opened. These relays, however, are slow to release and will remain in their operated position after the opening of their respective circuits for a length of time suiiicient to test the ten trunk groups by means of relays 310-319. By the time relays 329 to 325 release, either some primary selector and a trunk in the group accessible through it has been selected as evidenced by the operation and locking of some one relay in the group 310 to 319, or each trunk group has tested busy, either because all of the trunks are busy and no circuit closure waspossible for the group test, relays 310-319, or all the trunks but one in each group are busy, in which case no relay in the group 319 to 319 has operated since, as already described, relays 319-319 are marginal and do not receive sufficient current in a circuit which consists of the winding of a trunk test relay, its own winding and the resistance controlled through the contacts of the operated primary selector test relay. If no group test relay is operated, then, when relays 329 to 324 release, ground is applied through their respective back contacts toshort circuit each of the resistances associated with each of the relays 330 to 339, and since no group test relay has operated, no further operations take place. Therefore, when relays 329 to 324 release, resistances such as resistances 326-326 are short circuited and the group test is made again, only this time there is no resistance in series with'each of the relays 31B and 319.

Since it has been assumedthat all trunk groups each have but One trunk" available, the separate circuit of each of the group test relays 310 to 319 is now closed from solid ground on the contacts of relays 320-324, through the upper contacts of relays 330'339, appropriate contacts on the corresponding relays SEE-3E9, winding of each of the relays 310 to 319, respectively, back contacts of the relays 325 to 329, through the upper winding of the one relay in each trunk group appertaining to the idle trunk in that group, to battery on cam 358 of each of the associated idle trunks. The quantity of current flowing through each circuit of the separate relays 3'50 to 3'59 is now sufficient to operate each of said relays which, in turn, causes the operation of each of their corresponding locally controlled relays 35i3369. Previously traced circuits for relays 36B, 386 and 349 (assuming that the idle trunk in the group accessible through the IJ-primary trunk is that marked by 349) and 325 are now closed causing these relays to operate. With the operation of relay 386 and the locking of relay 360, all the other group relays 36i3t9 release, and the operation and locking of relay 3%, as already described, causes the trunk marked by said relay to be the one chosen for connection with the calling line. The trunk selected, therefore, is the 'il-trunk in the trunk group accessible through primary selector 0. Horizontal magnets I and Ill? are now operated for extending the line to the available trunk, while all of the relays associated with the line, start and sender group start circuits are released as previously described.

In the foregoing description it has been assumed that when the trunk groups are tested, their correlated sender selector groups all have available sender selectors and senders as evidenced, for instance, by the continuity of the separate circuits of each of the relays M5 to 3H! marking the sender selector group correlated with each of the trunk groups accessible through primary trunks 0 to 9, respectively. In the actual practice of the invention, however, this is not necessarily true as it is possible that, although free primary selectors may have available trunks, yet no sender in the groups appertaining to the trunk group accessible over said primary selectors may be available, in which event no trunk from such group can be selected. Under such circumstances, it becomes necessary to shift the selection of a trunk to other free primary selectors which have available trunks in the trunk groups accessible therethrough and also available senders in the group correlated with said trunk groups.

Thus, if primary selectors 0 and 9, both have available trunks and the sender group correlated with said selectors and marked by relays 3E5 to 355 have no senders or sender selectors available, the circuit for relay 315 is not completed since said circuit is completed through the normal contacts of the horizontal magnet of each sender selector and the normal contacts of relays 189-489 which are operated when all the senders available through said selector are also busy, there being one such relay such as relay 189 for each group of senders available through each sender selector. The failure to operate relay 355 precludes all possibility of testing the trunk groups accessible through primary selectors 8 to 9 and identified by relays 310 and 319 since, as already described, their respective circuits are controlled through the operated contacts of relay 3l5.

. But if, at the same time, any other'two trunk groups, say the trunk groups associated with primary selectors 4' and 5 do have available senders in the correlated sender group identified by-relay 3l9, then the circuit of this relay would be closed over a path'controlled over conductor 396, through the back contactsof all magnets of idle sender selectorsin the lth sender selector group and the back contacts of relays 189 and other similar relays which would be normal if all the senders available through the particular selectors are not busy, conductor 50!, to battery. Under such circumstances, a trunk would be selected from one of the trunk groups accessible through primary selectors 4 or 5, or through whatever other pair of primary'selectors hadan available sender and sender selector in the respective correlated trunk group which is nearest in electrical preference with respect to the selection of such sender selector groups as determined the chain preference circuit of relays 310-419. Once the available sender selector groups are determined, then the selection of the trunk groups is determined by the preference selection circuit of the group of relays-3l0+3'l9. Actual trunk group and trunk selections are then carried out as described above.

' Sender selection .back contacts of relay 100, back contacts of relay,

1m, winding of relay 102 to battery. Relay 500 operates, locks in a circuit extending from battery through its upper winding and bottom contacts, conductor 503, over the back contacts of the normal lock-out relays of other sender groups, such as relay 60B, conductor 6M, wind ing of relay Hill, back contacts of relay Till, to battery through the winding of relay Hi2. Relay ma operates and opens the operating circuit of all relays 50ll6ilt, so that, during the interval when relay 500 isoperated, no other lock-out relay in other sender selector groups can oper ate. Relay 500, over its top outer contacts, closes an obvious circuit for relay 50! which operates and partially closes the paths for ten groups of horizontal magnets JUL-Elfi to Shh-5H9, inclusive, one group for each one of the ten primary sender selectors of the 0 sender selector group, while over its top inner contacts, relay 5% partially prepares circuits to the vertical selecting magnets 52B--529, intermediate groups of vertical magnets groups 530-539 inclusive.

In the meanwhile, the operation of relay Hill closes a circuit extending from battery through the winding of relay 792, back contacts of relay i0l, front contacts of relay I00, winding of re-' lays NU-H9 in parallel, top inner contacts of relay 12ll--129, respectively, to ground. Relays Hli-H9 operate andremove the short circuit around the respective resistances 130-139. With the relay 501 operated, the winding of each test relay l40149 is connected to the test conductor of each sender selector in the sender selectorgroup in the vertical contacts of which terminate each of the trunks in the groups accessible through primary selector 0 and primary selector 9, as well as other trunk groups belonging to another main group of 100 trunks serving other main groups of subscribers lines. If any of the sender selectors in this group are busy with a connection between a trunkterminating thereon and a sender acces sible through said selector, such a selector will have ground on its sleeve conductonv Those which are not busy and, therefore, available, do not haveground connected to their respec: tive sleeve conductors and consequently cause separate circuits to be closed between the winding'of each horizontal magnet l05i9 and the winding of corresponding test relay in the group of relays Mil-149; a typical circuit being one between Imagnet 51 0 and relay 140 which extends from battery, through the winding of horizontal magnet 51%, Noni contact set of relay 56H, conductor 5D5,winding of relay 140 to ground, causing thereby the operation of relay 1% but not that of magnet 5H). In the same way, all other idle sender selectors cause the operation of their corresponding test relays. If it is assumed that the first or 0 sender selector is idle, then relay- 14:) is operated as just described and a circuit is further closed extending fromground through the outer contacts of relay 1&9, top winding of relay 120, back contacts of relay 150 to battery. Relayy12l3- operates, closes a locking circuit for itself over its bottom winding and bottom inner front contacts, back contacts of other relays such as relay 128 in a series chain, winding of relay 150 to battery. Relay 15B operates and opensthe operating circuit of all the relays 120-420.

In this connection, it will be noted that there is more than one relay 120 supplied for controlling the circuits relating tofthe-establishment of a-connection over a fl'prim'ary selector,.

such as for instance; 12ll1,and that a similar provision is made for each of the other selectors, The reason such as for instancyrelay 129 for this is due to the fact that the senders which are accessible through each of the sender selectors available:through the five sender selector groups, may likewise be multipled'for the use of other groups of sender selectors which are, in turn, connectible toother primary selectors serving entirely different groups of trunks.

separate sender group maynotbe justified in which event one sender group may be used in common by more than, say,. fivegroups of sender selectors. Each additional group of sender selectors, and the separate selectors therein corresponding to the numerical order of selectors in corresponding groups, would have a cutin relay such as relay 1201 01 129 .operated from the contacts of a corresponding test relay.

of the control circuit of said other sender selector group and its operation would then serve to connect the callingtrunk with a chosen sender by operating the horizontal magnets of the But due to the quantity of trafiic" which is handled by this other trunk group, a

126, winding of relay-168, back contacts of relay 110, through the winding of each of the respective relays in the group 180-180, to battery on the back contacts of the corresponding relays 181-18l. Relays 18l-18P 183---183 and similar groups of relays (not shown) are test relays each of which is connected to an individual sender available over similar sender selectors in the different groups and the operated or non-operated condition of each of the said relays marks the associated sender as busy or idle respectively. Each of the relays in the group 18I18l appertains to a sender in the first orll primary sender selector in each horizontal group While each of the relays in the group 183183 appertains to a sender in the last or 9th primary sender selector in each horizontal group.

Nowthe trunk circuit which has been assumed to have been marked for the connection, namely, the G-trunk, appears in the first or 0- group of ten sender selectors, which fact was indicatedbythe operation of relay 500. That istosay, by virtue of the fact that the connection of a selected trunk canbe extended to'a sender through any one of ten possible horizontal-paths, it may have accessibilityto any one of the-idle senders available through any one of said sender selector paths. And at the time the test of each horizontal path was made over a circuit connection between the horizontal magnet and its corresponding test relay (such as magnet 5H) and relay 1430 as already described) the number'of idle sender selectors was determined which, in turn, indicated that the selector testing idle wasfree. Consequently, the number of relays in the group 149-149 which was operated at the time the selector test was made indicatedthe number of such selectors that were idle. Each of these operated relays further closed a group test circuit through the corresponding group relays 160169, each with its own group of sender test relays, such as for instance, relay 169 with the group of ten relays 180-186 locally controlled through contacts of relays 18i18l for testing each of the senders in the first or B-primary selector, and relay 169- with the group of ten relays 182182 fortesting each of the senders in the last or 9th primary selector, while the same is true for the intermediate groups which are not shown.

If more than one sender is idle in each group, each of the separate group relays 160-169 will operate in parallel with the windings of the separate sender test relays in their respective groups which mark idle senders, one such circuit having been traced through relay 166 and relays 180-180 to battery on the-back contacts of relays Nil-18i The separate sender test relays, however, do not operate. When the group test relays operate, they close individual operating circuits for their locally controlled relays 184-4349, one of said circuits being traced from ground on the back: contact of relay 185, back contacts of relay 186, front contact of relay 160, lower winding of relay 184 to battery. Relay 184, and all-the others in the group- 184-184 which have had similar circuits closed, all, operate. Each of them will endeavor to close a locking circuit for itself from battery through its respective upper winding and top inner front contacts, through the top inner back contacts of other relays in the group which have not been so operated, said locking circuit extending in a through conductor 788; winding of relay 186 to ground on the back contact of relay 185. However, in view of the fact that the locking circuit of each succeeding relay is controlled through a back contact of the relay immediately preceding, only one of those operated can .lock and this relay will be the one whose locking cir cuit path is nearest to the winding of relay E86. If, therefore, relay 184 has operated in response to the idle condition of more than one sender in the group of senders available through the sender selector marked by the test relay M0, that is, the Q-selector, then, since relay 184 is nearest in electrical preference to relay I86, said relay 6'84 will lock in series with relay 7.86, the

latter relay also operating in said locking circuit. The operation of relay 785 opens the operating circuit of all relays in the group R k-18 which will release all those which had operated except relay 184. Thus, the available sender selectors are chosen according to a definite preference beginning with the zero selector and ending with the ninth.

After the sender selector has been marked as selected and the group of senders available therethrough has also been marked both by relay 184, if the sender selector chosen is the B-selector, an idle sender in the group accessible through that selector is then selected from among those that are idle. When relay 186 operates, it closes a circuit extending from ground on the contacts of relay 635, front contacts of relay 786, bottom front contacts of relay T84, bottom contacts of relay Till, top windings in parallel of all relays in the group l88l80 to battery on the back contacts of all corresponding relays in the group of relays l8i'i8l which are normal because of the idle condition of the corresponding senders they mark. The looking circuit of each of these relays is controlled through the bottom back contacts of the next succeeding relay in the group so that the operation of a preceding relay opens the locking circuit of the succeeding relay. Hence, if relay 18%) is operated, said relay will look over a circuit path extending from battery through its lower winding and front contacts, bottom outer front contacts of relay 72B, winding of relay T70 to ground on the back contacts of relay 185. Relay 780 looks and relay ill! operates in series with said locking circuit, the former opening the locking circuit of all other sender test relays in th group and the latter, by opening its back contacts, opens the operating circuit of all other test relays causing them all to release.

The sender in the group accessible through the marked sender selector is thus also chosen according to an electrical preference arrangement. If the sender marked by relay 189 had been busy and, in consequence, relay 180 would not have been operated, then the test relay or test relays of other idle senders would have operated and the one Whose test relay did not have its locking circuit opened by a preceding relay would look and cause the corresponding sender to be chosen.

When relay TH) operates, it closes a circuit extending from ground through its upper contacts, winding of relay 192 to battery. Relay 192 operates and extends the windings of all five groups of vertical magnets, to the contacts of the separate test relays of all senders. Hence, when relay 78% operates, a circuit is closed extending from ground through the top inner contacts of relay 180, No. contact set'of relay 792, conductor lall, winding of vertical magnet can to battery, and in parallel therewith, the winding .of each of the other vertical magnets in each vertical group, up to and including vertical magnet 540. All of said magnets operate, in preparation for closing through the selected trunk to the selected sender.

In. the meanwhile, the operation of vertical magnet Eli] closes a circuit extending from ground on its contacts, conductor 602, winding of relay lfll to battery and, in parallel with said winding,.topouter front contacts of relay 18 i, conductor 505, No. '0 contact set of relay i, winding of horizontal magnet 5H3 to battery and in parallel therewith to the windings of all other horizontal magnets in the selector including' magnet 550. Sincev vertical magnets 52%] and 5 58 are operated, thecross points of trunk 0 and the cross points of sender i] are closed through on the common conductors of the selector, thereby. establishing the required connection between trunk and sender.

When the trunk and sender are closed through, the usual operations in the sender will cause'ground to be connected to conductor 56! which will replace that on conductor 560 to hold the horizontal 'magnet'soperated after the selector control circuit has released.

The operation of horizontal magnet 5H) opens a contact in the number of parallel circuit paths of relay 3E5 so that when all of the sender se-' lectors in the il-horizontal group are busy, the continuity between conductors 399 and 5532 is broken as a signal that no senders are available throughthis group.

At the time'the circuit of magnet 5!!! was closed, another circuit in parallel with it was closed to the winding of relay HH, as already described. Relay it! operates, opens the circuit of relay lot and closes a holding circuit for relay 50! from battery winding of relay M2, the front contacts of relay 'llll, conductor 79! bottom contacts of relay 5M to the winding of said relay, holding said relay when relay 50B. releases at the time relay iii! operates. This is done in order to insure that the circuit connection to the horizontal magnets is long enough to allow for the send er to operate and connect ground to conductor 56! to hold said magnets. In the meantime, the release of relay Hill causes the release of relay 500 which, in turn, releases the vertical magnet 52!).

Further, when ground is connected to conductor 56! by the sender, a circuit is closed from said ground on conductor 56!, winding of relay "5'34 to battery. Relay 'lBi operates and closes a circuit extending from ground on its inner front contacts, top outer contacts of relay 686, top inner contacts of relay H0, winding of relay 85 to battery. Relay 185 operates, opens the circuit of relays H0 and I86 both of which release, in turn, releasing relays 192 and HM. The release of relay 192 .opens the circuit of the vertical magnets till-548 while magnet 6m, in releasing, opens the circuit of horizontal magnet 588 which is now held operated in'the'previously described circuit to ground on conductor 56!. It further releases relay ml which, in releasing, opens the holding circuit of relay 58! which also releases. The entire circuit is now restored to normal with the exception of horizontal magnets 5H] and 55B Which'are holding the established connection between the trunk and sender.

The relationship between sender selectors and sender groups accessiblethrough saidselectors contains the same reserve feature that is contained in the relationship between primary. trunk selector and trunk groups. The operation of the circuits with respect to these features will now be described. l i

It will be remembered that in extending the connection to an idle sender it has been tacitly assumed that (1) the El-sender selector in the group containing the marked'trunk was available; (2) that more than one sender of the group of senders connectible therethrough'were' idle. The first assumption was indicated by the operation of relay I40 series with the horizontal magnet 5H! of the 0sender selector; and the second condition was .evidenced by the operation of relays, in the group l8l'l8l which correspond to idle senders accessible through the t-sender selector. Obviously, as respects these two conditions there may be a variation in either orboth of these two factors; There may be, for instance, no available sender selectors in the 9 group which contains the-tentatively marked trunk although there maybe two or more senders available in each of the jsender selectors comprising the group. Also, the sender selectors mayrall beidleand yet .the selectible senders which, may be reached through them, may all be busy;.:or again, while an idle sender selector andan idle connectible sender may both be simultaneously'available yet there may be other idle sender selectors connectible to the tentatively marked trunk which have more thanone sender available. .Each of the last two conditions produces'its own reaction by virtue of Which the calling line is transferred,

respectively, to another sender selector in the" same group connectible'to the tentatively marked trunk, or to another trunk which is connectible tothe same main group of senders but through an entirely difierent group of sender selectors than the one associated with the tentatively marked trunk. In the latter case the previously marked trunk is dropped and another trunk is marked which is connectible to the senders by way of another group of senderselectors. Each of these separate conditions will now be described in detail.

Sender selectors all busy If the senderselectors in the horizontal group accessible to the tentatively marked trunk are all busy, such as for instance, the ten sender selectors of the fi-horizontalgroup accessible to the trunk G, then horizontal magnets 5iil5|9 are all-operated and the circuit of relay 3l5 is broken at each of the pair of contactscarried by said separate magnets. Consequently when the line start control circuitshown in'Fig. '3 operates to test the various sender selector groups correlated with the various trunk groups, the relay 315 which tests the Ei-sender selector group will have its circuit. opened. The failure of the relay 3i5 to operate under such circumstances is a signal to the line start, control circuit that a trunk in the group correlated to the fi-horizontal group of sender selectors can not be marked since there are no sender selectors available which can be used to establish a connection' between a trunk which terminates on said sender selectors andthe senders accessible therethrough. The line start circuit, of course, is at the same time testing" other sender selector groups through the association of other relays similar to 'j3l5,:such 'as relays"3l6--3l9tcorrelated to other four sender selector groups and which have their circuit. closures controlled through the back contacts of the respective horizontal magnets of said sender selector groups. The line control circuit then proceeds to mark a trunk in a group correlated to a sender selector group whose marking relay in the group 3l5--3l9 has operated. If more'than one trunk group shows a simultaneous availability of correlated sender selector groups, then the line control ,circuit'will' mark a trunkin a preferred group as previously described 'On the other hand, the sender selectors in the G-sender selector group may all be idle but the senders accessible therethrough may all be busy. This condition is evidenced by the fact that as each sender becomes busy, its test relay in the control circuit is operated. When, therefore, all the senders in the group accessible through the B-sender selector of the H-sender selector group are busy, all of the sender busy relays corresponding. to senders accessible through said selectors, and consisting of relays ESL-78H are all operated, each over a circuit of the character already described for relay 78!. It will be observed, however, that the circuit of the. marking relay 3E5 is not only controlled through paralleled contacts on the horizontal magnets 5l65l9 but is further carried over the back contacts of relays 189-189. Consequently, when the circuit of relay M5 is being established and itscircuit path is continuous through the back contacts of the horizontal magnets 5lil 5i9, then, if the senders available through the ll-sender selectors are all busy, relay 789 Willbe operated as already described, and the path joining conductors 399 and 532 at the back contacts of this relay is opened. The circuit of relay 3l5, however, has other parallel paths through'the other horizontal magnets, namely through magnets 5H'5l9. If, therefore, any sender group accessible through the corresponding sender selectors have more than one sender available therein, the group relay T39 corre sponding to the group of busy relays similar to TBI-lBl will be normal, whereupon the circuit is then completed through the back contacts of the horizontal magnet of'lth'e' idle senderselectorand the. normal contactsiof the relay T39 allocated to the group of relays similar to relays Till-13i corresponding-to the sender group available through said sender selector. As each sender group becomes busy and itscorresponding group relay "(8%! is operated, another parallel path of the circuit or relays H5 is opened, so thatwhen all groups .are busy, all'paths are opened and relay "3H5 can not operate. Hence, if all selectors in the group are busy, all paths are opened at the contacts of the horizontal magnets'of-said selectors; if all sender groups available through said sender selectors are busy,

saidpaths are broken at the appropr ate back contacts on the operated group relays 7B8'l89, and if all selectors and all sender groups accessible through said selectors are busy, then all paths are broken and relay 3i5 cannot operate. Under such circumstances, the line control circuit then proceeds tomark a trunk in a group correlated to one of the other four sender selector groups, each of whose marking relay 3H5- 3l9' may have operated.

If, on the other hand,,all selector groups are busy and all sender groups are busy simultaneously,,the circuit paths of the five marking relays '3l5'3l9 are 'all opened'and none of these 

